A “dark fiber” that forms an optical transmission path in a system using an optical submarine cable is a pair of optical fibers unconnected to or unequipped with a submarine line terminal equipment (hereinafter, occasionally abbreviated as SLTE) at the initial stage, each being provided with optical repeaters. For monitoring such a dark fiber, a submarine optical repeater-monitoring dedicated device (for example a Coherent-OTDR measuring instrument (external measuring instrument by OTDR applied to an optical amplifying system using Coherent light)) independent of an optical submarine cable accommodating a pair of optical fibers as a main signal transmission path connected to or equipped with the SLTE has been heretofore employed.
A dark fiber monitoring system as related art is depicted in FIG. 7, in which an SLTE 1 in a terminal station 100 is connected to an SLTE 1 in a terminal station 300 through a pair of main signal optical fibers LF. Between the terminal stations 100 and 300, dark fibers DF1-DFn each composed of a pair of optical fibers are connected in parallel with the above optical fibers LF. To the dark fibers DF1-DFn, the SLTE 1 is not connected.
In operation of a main signal route of the SLTE 1 in the terminal station 100, an optical main signal is outputted from a main signal transmitting portion (Tx) 3, division multiplexed at an optical wavelength division multiplexer (WDM) 4, amplified at an optical amplifier 5 and sent to a variable optical attenuator (VOA) 6 that is a modulator.
On the other hand, a monitoring signal generator 9 having received a monitoring request command CMD from a network management system (NMS) connected to the SLTE 1 through a maintenance processor (MP) 8 converts the monitoring request command CMD into a monitoring (supervising) signal SV to be provided to the variable optical attenuator 6. This variable optical attenuator 6 amplitude modulates the optical main signal from the optical amplifier 5 with the monitoring signal SV from the monitoring signal generator 9 to be sent out to the optical fiber LF.
In the optical fiber LF between the terminal stations 100 and 300, a plurality of optical repeaters (REP) 200_1, 200_2, . . . , 200—j, 200—k (hereinafter, occasionally represented by a reference numeral 200) are set, where the optical repeaters 200 are respectively provided with a module (subsystem) 20_0, which includes an optical amplifier 21 in the direction from the terminal station 100 to the terminal station 300 and an optical amplifier 22 in the opposite direction (see FIG. 3 as undermentioned).
The optical output signal, superposed with the monitoring signal SV, outputted from the variable optical attenuator 6 in the SLTE 1 of the terminal station 100 is sent for the SLTE 1 in the terminal station 300 through the optical repeaters 200 in the main signal optical fiber LF, as well as responded by the optical repeaters 200 and sent back to the SLTE 1 in the terminal station 100.
In the SLTE 1 of the terminal station 100, the optical main signal after passing through the optical coupler 11 and the optical amplifier 12 is demultiplexed per wavelength by a wavelength division demultiplexer (WDM) 13 and then received by main signal receiving portions (Rx) 14.
On the other hand, from the optical main signal branched at the optical coupler 11, the monitoring signal SV returned from the optical repeaters 200 is extracted by a monitoring signal extractor 15 and sent to the network monitoring system 2 through the maintenance processor 8.
The above signal flow is similarly applied to the optical main signal sent from the SLTE 1 in the terminal station 300 to the SLTE in the terminal station 100 through the main signal optical fiber LF, so that the optical main signal is returned to the SLTE 1 in the terminal station 300 through the optical repeaters 200.
It is to be noted that the monitoring signal SV sent toward the optical repeaters 200 will be hereinafter occasionally referred to as a monitoring request signal (SVC) and the monitoring signal SV returned from the optical repeaters 200 will be hereinafter occasionally referred to as a monitoring response signal (SVR).
The monitoring response signal (SVR) may include the following signals:
(1) Optical input power of the submarine optical repeaters;
(2) Optical output power of the submarine optical repeaters;
(3) LD bias current.
On the other hand, to the dark fibers DF1-DFn unconnected to the SLTE 1, by connecting the optical repeater-monitoring dedicated devices 30 respectively provided in the terminal stations 100 and 300 the optical repeaters 200 are monitored. Each of the optical repeaters 200 in the dark fibers DF1-DFn is provided with “n” pieces of modules 20_1-20—n respectively corresponding to the “n” dark fibers. The optical repeater-monitoring dedicated devices 30 check which of the dark fibers DF is/are maintained normal, and also check up to which optical repeater the main signal transmission path is maintained normal based on the dark fibers having been checked to be normal.
As one example of the above noted related art, there is a dark fiber supervisory testing device and apparatus, in which a test optical signal is made incident onto the dark fiber from a light source in a station for a facility rental enterprise via a multiplexer/demultiplexer section at one end, the optical signal is received at the other end of the dark fiber in a user building a via multiplexer/demultiplexer at the other end, a result of the received light is returned to one end of the dark fiber, received at one end via the multiplexer/demultiplexer at one end side, a control/notice unit calculates the optical transmission characteristic of the dark fiber and informs the user about it via a user unit (see e.g. Japanese Laid-open Patent Publication No. 2003-244080).
The related art as depicted in FIG. 7 has required a dedicated device for exclusively monitoring dark fibers forming optical transmission paths, resulting in a high system cost and complicated managements.
Also, a Coherent-OTDR measuring instrument (testing device) as one example of the optical repeater-monitoring dedicated device is one used for a fault search, so that it is not suitable for a full time monitoring over 24 hours and 365 days.
Furthermore, in case of a plurality of dark fibers used it is disadvantageous that the optical repeater-monitoring dedicated devices are required to be manually switched and connected to the dark fibers and therefore can not be remotely operated, failing to monitor all of the optical fibers by a simple operation.